1. Field of the Invention
The present invention relates to a driving system for electronic devices, and particularly relates to a driving system for lamps.
2. Description of the Prior Art
Conventional back light modules of a flat panel display always utilize at least one CCFL (Cold Cathode Fluorescent Lamp) as a light source. However, since the number of lamps follows the size of the flat panel display, the luminance and uniform degree are severely requested. Furthermore, each CCFL may have different characteristics, and therefore may have different passing currents and luminance even though the same voltage is provided to each CCFL. Thus, a current balancing mechanism is needed.
FIG. 1 and FIG. 2 illustrate prior art current balancing circuits for lamps. As shown in FIG. 1, the current balancing circuit 100 includes a power stage 101, a transformer 103, a control circuit 105, and capacitors 107 and 109. The power stage 101 is utilized to provide an AC voltage V1 according to a DC voltage Vin. The transformer 103 is utilized to transform the AC voltage V1 to an AC voltage V2. Thereby the lamps 111 and 113 can obtain currents according to the AC voltage V2. The control circuit 105, which is always a PWM control circuit, is utilized to control the power stage according to the currents I1, I2. As described above, even though the lamps 107, 109 are provided with the same voltages, they may have different passing currents I1, I2 due to their different resistances.
FIG. 2 illustrates a prior art current balancing circuit 200 for lamps. The current balancing circuit 200 has similar structures with the current balancing circuit 100. The most apparent difference is that the current balancing circuit 200 has two power stages 201, 203, which provide currents to lamps 205 and 207, respectively. As shown in FIG. 2, the two power stages 201, 203 make the current balancing circuit 200 more complicated than the current balancing circuit 100. Accordingly, the current balancing circuit 200 is hard to be controlled by the control circuit. Besides, such structure also results in an increase of cost and space.